A wide variety of methods have been used over the years to purify water to produce potable water from a contaminated water source or to disinfect the outer surfaces of foods such as fruit and food packaging. Purification of water is becoming an increasing problem because of the increasing levels of contamination in many water sources. Similarly, disinfecting products such as fruit, vegetables and the like to eliminate microorganisms that can contaminate the products during growing handling and shipping, particularly with long distance shipping of such products.
Municipal water supplies have generally been purified by a combination of methods, including filtering the water and treating the water with chlorine. Other agents are being used to varying degrees, such as treatment with ozone gas, administration of various types of radiation, including ionizing radiation, thermal methods and the like.
Ultraviolet light (UV) is an effective disinfecting or sterilizing agent with few, if any, side effects. A UV radiating plasma is highly effective for treating surfaces, such as packaging and for fluids such as water. For example, Tensmeyer in U.S. Pat. No. 3,995,921 describes a process using a focused laser beam to generate UV with a pulsed plasma source to disinfect the interior of a container. However, the method disclosed is not useful for treating flowing fluids, such as water.
Copa, in U.S. Pat. No. 4,265,747, describes use of a laser beam to generate a UV radiating plasma for treatment of a fluid. However, its applicable to small volumes only. Hiramoto in U.S. Pat. No. 4,464,336 teaches the use of a wall stabilized flash lamp for sterilizing microorganisms. It is apparent from the Hiramoto disclosure that he uses a "wall stabilized" flash discharge lamp. In the wall stabilized mode of operation a plasma column expands and fills the entire quartz lamp during a capacitive discharge. The resulting operation of the system, and the pulsed light produced, is then dominated by the walls and the confined plasma. This prevents operating at higher temperatures, voltages and longer pulse widths as would be desirable to provide shorter, more effective UV wavelengths. A similar wall stabilized flashlamp plasma concept, having the same problems, is described by Dunn in U.S. Pat. No. 4,871,559 for treatment of food packaging.
Thus, there is a continuing need for improved systems for treating water and other fluids and various surfaces with UV that overcomes the limitations of wall stabilized flash discharge lamps, provides short pulse durations at shorter UV wavelengths and is highly effective in disinfecting a continuously flowing stream of water or other fluid.